What Are They: Custom In Ear Monitors (IEMS or CIEMS) are devices used by musicians, audio engineers and audiophiles to listen to music, hear a custom mix of vocals and instrumentation usually during a live performance. 

Tekhne + Logia (greek) is the "systematic treatment of an art, craft or technique". Or at least this was the working definition in the 1600s. Today's definition has been modified a bit and reads: